Doc's 'Future Plutonium Thoughts' make no sense

As Doc is preparing for his trip to the future, he 'realizes' he can't get back to the past without 'extra plutonium', and remarks that he must be out of his mind.

Now, did he think this through? Does he think time DESTROYS plutonium? Wouldn't the plutonium (from his perspective, being stashed somewhere safely) be WAITING for him in the future? I mean, sure, something COULD have happened to it in 25 or 30 years, but at the same time, plutonium would still have existed, so the VERY least, he could still have made the same kind of 'deal' with the same type of people (I don't want to say the T-word here) and have gotten more plutonium that way.

What did he think would happen to his plutonium box if he didn't take 'extra plutonium' with him? He didn't know at that point in time that the people would come to avenge his 'shoddy case full of pinball machine parts-prank', so he was possibly under the impression that Marty could've stashed the plutonium somewhere safely, where he probably could get to it in the future even if he forgets to bring extra with him.

It's the trouble with fourth-dimensional thinking, I guess.. (although in reality, time is not a dimension, and higher dimensions are not necessarily mathematical, but frequential (?) ones, meaning astral plane and so on - I think 'etheric' is not its own plane, but more like a conduit inbetween the astral and the physical, but that's a bit beyond the scope of this post)

Also, why did he plan to time travel alone, considering how much they time travel together in other movies, and even bring Jennifer with them in this one? It makes no sense for Doc to just travel into the future by himself (or just with Einstein), when Marty is RIGHT THERE, and has been a solid part of the most important experiment of his whole life. He's just gonna abandon him suddenly and embark on the 'trip' alone? What? Why? Isn't Marty his friend, doesn't he DESERVE to see the future after all that help and videotaping?

Wouldn't he WANT his trip to ALSO be documented, since he's so meticulous about wanting everything else leading to that be documented and videotaped?

I don't get his thought process here at all, not about the plutonium, not about Marty, and not about the documentation process. Why document the first experiment, but not the grand trip? Don't people LOVE taking travel videos even nowadays?

It makes no sense.

P.S. As a sidenote, obviously that plutonium amount is extremely limited. He can basically make a 'handful of trips', and then he has no more plutonium. Does he have a contingency plan for this? I know he ends up solving it due to future tech, but he doesn't know that's going to happen.. OR, is that his plan all along? Because if it IS, then why would he EVER WORRY about not bringing extra plutonium? AGAIN his plutonium-related thoughts make no sense..